The invention relates to a radial bearing of the sliding-bearing type, in particular for use in centrifugal pumps, with a bearing sleeve arranged on a rotating shaft part and designed to transmit torque, the bearing sleeve being arranged rotatably within a bearing bush, and a gap for a low-viscosity lubricating medium being located between the parts sliding on one another.
Various embodiments of shaft mountings are known in centrifugal pumps, different bearing materials being used, depending on the feed medium. Ceramic bearing materials have proved particularly advantageous in bearings which are lubricated with a low-viscosity feed medium, for example with water or alcohol. A disadvantage of ceramic bearings of this type is their sensitivity to overheating due to deficient lubrication and jolt-like loads. Such sensitivity occurs in the region of mixed friction, for example when the sliding faces touch one another as a result of radial loads which are too high. At the same time, thermal-stress cracks may be formed within the ceramic due to an excessive local introduction of frictional heat into the ceramic surfaces. This gives rise to the risk of local pronounced material overload, with the result that cracks or spalling occur on the ceramic part and consequential damage is to be expected.
An additional load acts on such a mounting consisting of a stationary bearing bush and of a rotating bearing sleeve, when an angular offset arises between these parts. For example, it is known from DE-A-1528640, in such mountings, to arrange the bearing bush elastically relative to a housing by means of O-rings, in order to achieve easy moveability.
In the case of large multi-stage pumps or long shafts with bearings arranged between them, for example in large feed pumps, borehole shaft pumps or cantilevered mountings with a shaft overhang on one side, inclinations or deflections of the shaft occur as a result of the forces which prevail during operation. As a result, the bearing sleeve rotating together with the shaft likewise assumes an inclination. A one-sided bearing load resulting from this constitutes a further risk to ceramic bearings which are sensitive to impact or jolt loads.
The problem on which the invention is based is, therefore, to develop a shaft mounting which uses break-sensitive materials and which allows an angular offset occurring in the mounting.
This problem is solved in that the bearing sleeve consists of a carrying element and of a bearing element fastened to the latter, in that a fibrous structure arranged predominantly in a matrix-like manner and having ceramic particles or carbon particles arranged in it forms the bearing element, in that a ceramic or carbon matrix forms the fibrous structure, in that the carrying element is provided with different wall thicknesses in the region of bearing contact of the bearing element, and in that the carrying element is designed to be equal to or longer than the bearing element.
As seen in longitudinal section, the carrying element has a wall-thickness profile which is at a maximum in the middle region and is configured to decrease from the latter toward both sides. The ceramic or carbon-containing bearing element having a predominant matrix-like fibrous structure is pressed with a shrink fit or press fit onto the carrying element. The dimensions and tolerances of the individual parts are selected such that, under the effects of temperature, the bearing element remains firmly connected to the carrying element. The combination of such a bearing element with a carrying element having elasticity affords the advantage that, when an angular offset occurs, the two parts react in an elastically resilient manner and therefore the risk of a break on the bearing element is prevented. In contrast to a monolithic component, the fibrous structure, which is arranged in a predominantly matrix-like manner and is designed as a ceramic or carbon matrix into which ceramic or carbon particles are embedded, has resilience with respect to bending loads. At the same time, depending on the desired bearing pairing, both ceramic particles and carbon particles may be arranged in a carbon matrix. The same also applies to a ceramic matrix.
Thus, a property more resistant to tensile stresses occurring under bending loads can be produced on such bearing elements which are per se break-sensitive and are made as sintered parts. The tensile stresses occurring under bending load in a sleeve cross section of such a bearing element and risking a break are improved by the factor 10, as compared with a pure sintered material, by means of the fibers arranged in a matrix-like manner. The angular deviations of the mounting can consequently be compensated.
If the bearing element is connected to the carrying element by means of a shrinkage connection, the shrinkage forces of the bearing element cause the formation of a slightly convex shape in the assembled state. Since, according to one embodiment of the invention, the carrying element has a substantially smaller wall thickness in the region in which the ends of the bearing element are located than in the middle region of the latter, the shrinkage forces cause a reduction in diameter in the region of the smaller wall thicknesses. A shrunk-on bearing bush with its fibrous structure composed predominantly in a matrix-like manner, for example consisting of silicon carbide fibers or carbon fibers, therefore has a slightly convexly formed surface which is conducive to the compensation of angular deviations of a shaft mounting equipped with it. The combination of the bearing element equipped with a matrix-like fibrous structure with the carrying element, the shape of which leads to a spring characteristic curve about the radial axis, ensures an elasticity which compensates angular errors. In the case of pressed-on bearing elements, only the elastic resilience ensures the compensation of angular deviations.
Further embodiments of the invention are described in the subclaims. The feature whereby the end faces of the carrying element project beyond the end faces of the bearing element reduces the occurrence of stress peaks and allows the spring characteristic curve to be influenced positively. By virtue of a free space being arranged between the shaft and the carrying element in the region of one or both end faces of the bearing element, space is provided for the elastic compensation of angular errors.
The carrying element and the bearing element fastened to it form the rotating part of the radial bearing, the bearing sleeve. In a middle region of the bearing element, the carrying element has a wall-thickness maximum, the dimensions being selected such that reliable force transmission is thereby ensured. Furthermore, the region of the wall-thickness maximum serves for receiving means transmitting rotational movements between the shaft and carrying element. Wall-thickness minima are provided on the carrying element in the region of the end faces of the bearing element. These minima are conducive to the spring effect of the carrying element and to the formation of a convex shape of the bearing face.
At least one thick-walled end portion led up to the shaft may be provided in the region of the end faces of the carrying element, this end portion being arranged at a distance from the end face of the bearing element. In between, the wall of the carrying element is designed as an elastically resilient thin-walled portion. The thick-walled end portion arranged in the region of the end face of the carrying element may also have torque-transmitting or force-transmitting designs. For the bearing contact of the bearing element, the carrying element may have an end-face bearing surface.